Typo in fold engine?
- From: Thomas Schilling <nominolo@xxxxxxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Tue, 17 Jul 2012 17:13:03 +0100
Mike,
I was looking through the code for the FOLD engine of LuaJIT and it
looks like there's a typo in this code:
LJFOLD(SUB SUB any)
LJFOLDF(simplify_intsubsub_leftcancel)
{
if (!irt_isnum(fins->t)) {
PHIBARRIER(fleft);
if (fins->op1 == fleft->op1) { /* (i - j) - i ==> 0 - j */ <--- HERE
fins->op1 = (IRRef1)lj_ir_kint(J, 0);
fins->op2 = fleft->op2;
return RETRYFOLD;
}
}
return NEXTFOLD;
}
I think it should read "fins->op2" instead of "fins->op1". As it
stands the test can only evaluate to true if we have an IR instruction
x = SUB x y
which is impossible in SSA. So it's not a bug, just a (tiny) failure
to optimise.
I tried to come up with a test case, but since this is an integer
optimisations its somewhat hard to trigger as only loop counters seem
to be narrowed to integers.
Patch is attached.
/ Thomas
Other related posts:
